A further continuously variable ratio transmission of the toroidal race rolling traction type is described in GB-A-2108600. A two regime continuously variable ratio transmission system, using a toroidal race rolling traction transmission unit and having another connection between the variator and the two epicyclic gear trains, is known from GB-A-2100372.
In such multi-regime transmissions, each regime is selected by a clutch which is engaged by the application of hydraulic fluid pressure as required by the transmission control software. Each regime covers part of the overall ratio spread of the transmission and a small degree of overlap is provided to facilitate switching from one to the next.
On a typical car transmission, low regime provides full reverse, through synchronous ratio to an approximation of second gear forwards and high regime provides an approximation of second gear forwards to full overdrive.
The transmission is so “geared” that changing from one regime to the next requires no change in the ratio of the variator. However the shift process occurs towards one or other of the limits of variator ratio, thereby making best use of its capabilities. Consequently, continuous acceleration through a regime change requires the variator ratio to be moved towards the appropriate limit before the shift but to be reversed thereafter. Since switching the control operation of the variator inevitably introduces a period of diminished capability alternative drive means must be provided during the process.
The provision of a degree of overlap between regimes allows the transmission to reach a finite ratio where simultaneous operation in both regimes is possible (referred to as synchronous ratio). At this point both clutches can be engaged without slip and power is transmitted, albeit at fixed ratio, independently of the variator.
The shift sequence is intended to produce an imperceptible regime change, without interruption to the vehicle's acceleration. It comprises the following operations:
(a) Fill and “lock” the next regime clutch. The clutch is applied by a piston and cylinder that takes a finite time to fill. Sufficient pressure must be applied to the piston to prevent slip but if applied before or after synchronous ratio is reached, will produce “shift shock”.
(b) Reverse the control force applied to the variator. This will reverse the power transmitted by the variator. However the two locked regime clutches will ensure power transmission. Fixed ratio operation for a short period will introduce a small but imperceptible engine speed change.
(c) Release the unwanted regime clutch Once the appropriate control force is applied to the variator, it and that regime clutch provide sufficient means for power transmission.
The most difficult part of the process is the engagement of the next clutch. The finite fill time requires that the process be started before the transmission reaches synchronous ratio. The degree of prediction involved requires accurate calculation of the rate of change of transmission ratio plus precise control of the clutch apply pressure.
Currently the engagement of the clutches is achieved by means of a two-stage (or “soft fill”) strategy in order to reduce sensitivity to the accuracy of the fill prediction. This involves filling the clutch at a pressure just capable of closing the clutch plates, thus avoiding shift shock if the fill is completed away from synchronous ratio. Subsequent switching of the clutch to high pressure in order to firmly engage the clutch plates can therefore take place virtually instantaneously and thus the decision as to when to engage the clutch can be based on the actual, rather than the predicted, ratio of the transmission.
One disadvantage of this approach is the inevitable increase in the clutch fill time resulting from the lower soft fill pressure. This introduces two issues.
Firstly, the elapsed time required to complete a regime change is unduly long. The need for best economy requires the engine to be operated at the lowest practical speed. Light throttle cruise is therefore provided with the engine at or close to idle. Acceleration demands then require an accompanying engine speed increase, which is likely to provoke a regime change into low. Protracted shift times are then perceived by the driver as response delays.
Secondly, it is difficult to predict accurately the correct time to engage a clutch. Lengthening the fill period requires an earlier shift initiation. Changing traffic conditions are then more likely to invalidate the prediction.